Different Types of Colour
The term colour depth refers to the number of bits used to represent the colour of a pixel in an image, this is done by using different combinations of the three colours Red, Green and Blue. Furthermore to create a white colour all the RGB levels are set to 100% where as to create a black colour all the levels are set to 0%.
A 1-Bit image is also known as a black and white image or monochromatic as the number of possible colours for each pixel is only 2, white or black. However this does mean the the files are usually very small in file size.
12 bit offers a slightly wider range of colours with four bits used to represent each of the RGB colours there are now 4,096 different colours, this is still however very limiting and is mostly used for mobiles and other devices with limited colour displays.
True or 24-Bit Colour
True colour can replicate near every colour the human eye can see, it uses twice the number of bits per colour as 12 bits meaning that there are 256 levels for each and over 16.7 million in total. This in the currently used system for storing coloured images on modern computers.
32 bits still uses the true or 24 bits system how as most modern computers now have a 32/64 bit word size the extra 8 bits are necessary but either ignored or used for an alpha channel.
Image File Sizes
Its is possible to calculate the true minimum size of a bitmap image file when you have its resolution and colour depth of the image. For example 1920 X 1080 X 4 bytes (or 32 bits) is equal to a file size of 8,294,400 bytes = 8,294.4 KB = 8.3 MB. However this can vary based on the fact that the resolution and colour depth need to be stored in the file header.